This invention relates to coating adhesives onto a substrate, and more particularly to roll coating rubber-to-metal adhesives onto a metal substrate.
In a number of industries, such as the automotive industry, the use of rubber coated metal parts in widespread. This is due not only to the resilient and damping properties of natural and synthetic rubbers, but also to their ability to coat and protect metal parts from exposure to the environment. Such rubber coated parts may find use as bushings, engine mounts, shock absorber mountings, and the like. These parts must be able to withstand such adverse environmental conditions as extreme heat and cold, salt sprays, corrosive atmospheres, chemicals, oils and solvents.
Because of the poor bonding obtained with straight rubber to metal bonds, adhesives have been developed which can bond metal parts to rubbers and withstand extremely adverse service conditions without bond failure. However, prior methods utilized to coat the metal parts, particularly hollow tubular metal parts, with adhesive are both inefficient and environmentally unsafe.
The most commonly used prior art method is spray coating in which a solvent based adhesive is sprayed from a nozzle or plurality of nozzles onto a rotating part, and the spray coated part is heated to drive off the solvent. This method is extremely inefficient in that only about 30% of the total adhesive sprayed is deposited on the part. The remaining 70% is lost or can be recovered only with great difficulty.
Moreover, since most if not all of the commercially available adhesives have a volatile organic solvent base such as benzene, toluene, xylene, methyl ethyl ketone, or methyl isobutyl ketone, their vapors must be controlled both to protect workers applying the adhesives and to comply with ambient air standards promulgated by regulatory agencies.
Likewise, dip coating of metal parts in an adhesive solution also has serious drawbacks. Dip coating is inefficient because the entire part is coated with adhesive material when in many cases only a portion of the part actually needs to be coated. Additionally, there may be a buildup of excess material on the edge of a part when it is withdrawn from the adhesive solution. This excess material may contribute to bond failure between the rubber and metal at a later time.
The use of prior brush or roller coating methods present problems of obtaining uniform coatings with no gaps or tears in the adhesive on the coated part. In many cases, the volatile solvent used in the adhesive coatings will attack and crack or embrittle rubber rollers. Use of steel rollers has not been found suitable because of transfer problems from the steel roller to the part to be coated.
Accordingly, the need exists in the art for a reliable and efficient procedure for applying a uniform adhesive coating to a metal part.